This disclosure is directed to an improved Vacuum Assisted Resin Transfer Molding (VARTM) process.
Resin Transfer Molding (RTM) provides a cost-effective fabrication method for the manufacture of polymer composites. In a traditional RTM process, catalyzed thermosetting resin is injected into an enclosed metal mold containing a previously positioned reinforcement preform. The preform is compacted to the specified fiber volume fraction when the matched metal mold is closed. The resin wets out the fiber until the mold is filled, and the part is then cured inside the mold.
RTM offers several advantages over other composite fabrication methods such as autoclave and compression molding of prepreg tape laminates. First, high fiber volume fraction composites can be fabricated with low void contents. Second, parts with highly complex shapes can be molded by incorporating many components into a single preform. This helps to reduce the cost and weight of the structure. Third, hand lay-up of prepreg tape is eliminated. Production rates are increased and operating costs are reduced. Finally, RTM is a closed mold process that reduces workers' exposure to harmful volatiles, i.e., styrene, associated with many of the room temperature processing resins.
Composites are fibrous materials infused with a tough and durable plastic. Composite material is a technology that is being used to make many items stronger and lighter. These materials are being incorporated into the armor of combat vehicles, weapons, and soldiers, for example. Developing a method that is fast and reliable is critical. A controlled setup is desirable in order to get optimized infusion. One process in which infusion is optimized is “Vacuum Assisted Resin Transfer Molding” (VARTM).
The VARTM process is an increasingly popular type of Liquid Composite Molding. In VARTM, resin is drawn into a mold by vacuum pressure and distributed evenly along a line in order to impregnate fibers. Generally, there are four steps in the process: First, the fabric preform is cut to a certain size, then it is put inside a one sided mold. Second, a bagging film is positioned around the fabric and adhered with tacky tape to the mold surface. Third, the resin is drawn into the mold and left until the fabric is saturated. Finally, the resin is cured then the final part is extracted from the mold.
As one example of this process, a vacuum pulls resin in from a feed tube to distribute it evenly into the preform. There are several different steps that must be followed in order to run a VARTM infusion process. A selection of materials that will be infused must be acquired. For example, twenty-four plies of glass and eleven plies of IM7 graphite may be stacked onto each other with the IM7 on the bottom. This creates the preform that will be infused. Another option for materials is a preform piece which may be held together by a binder.
The preform may be placed on a steel plate that has already been coated with several layers of a substance that deters two substances from bonding together, e.g., “Frekote”. This is important so that the finished panel does not stick to the steel plate and is easily accessible. To also help in this regard, a sheet of peel ply may be placed between the preform and the plate. The peel ply is a highly permeable fabric allowing resin to flow through it, but not hardening with the preform. When the process is finished, the peel ply allows the preform to have a uniform texture. The next step is to place another sheet of peel ply on top of the glass, so the vacuum bag does not stick to the finished composite panel. A layer of SCRIMP (Seemann Composite Resin Infusion Molding Process) distribution medium may be positioned on the glass. This enables the resin to travel at a higher speed so that it creates a sharper driving force to introduce resin into the preform.
Tacky tape may form the perimeter around the entire preform. It may be placed about two to three inches wider than the preform to give room for tubing and bagging. A piece of tubing, which has been cut in a spiral shape, may be placed at the back of the preform. Another piece of tubing, which is connected to the vacuum pot, is inserted into the coiled tubing. The vacuum pot is a sealed bucket that collects any excess resin that comes out of the preform. This is important because the vacuum pump could be ruined if resin enters the vacuum line. A feed line is then installed at the front of the piece. These lines may be made airtight by wrapping tacky tape around the edges that cross the previously made tacky tape perimeter.
The final step in the setup is the addition of the vacuum bag. The bag may be adhered to the tacky tape and positioned around the tubes. When there is excess bag in an area, an “ear” may be formed with tacky tape to guarantee that the bag will be airtight. Maintaining an airtight enclosure is a crucial step because any small holes could cause loss of a full vacuum and degrade the VARTM process.
During the VARTM process, checking the vacuum level is standard procedure especially when a large part is being made. Any air leakage will decrease the part quality and the part will be most likely be discarded.
Several conventional methods of leak detection in vacuum infusion processes have existed for some time, and have been used in the large scale manufacturing of industrial products. Current techniques of leak detection include leak isolation and sound magnification, both of which have disadvantages.
The primary leak isolation method used in industry and research today is simple, and is performed by vacuuming the air out of the mold, then clamping off the gates. If the vacuum pressure level has not decreased after a predetermined amount of time, the mold is considered to be free of air leaks. The disadvantage to this method is that it only able to indicate if there is or is not a leak, and does not specify the location of any leak.
Sound magnification is also simple and may be used to find the location of a leak. The method utilizes a microphone and amplifies the sound to a set of speakers or headphones, and any leak which is producing a sound should be detectable. The main disadvantage of sound magnification is that does not work well, particularly in noisy industrial environments, which is the principal reason it is not commonly used in industrial composites manufacturing.
What is needed is a simple and inexpensive method and system to detect air leakage in a Vacuum Infusion Process.